There’s a reason that the Missouri River seems so high in Omaha.
Runoff in the upper part of the watershed is on pace to be the third-highest amount in 120 years of records.
A series of dams controls that runoff, and because there’s been so much water this year, the Army Corps of Engineers has opened the gates of those dams wider than usual.
Additionally, the stretch of the river immediately downstream of the dams saw record runoff in September and October.
As a result, river levels have been up.
At the current rate, runoff is expected to total 163 percent of the average by the end of December, according to the corps. Amounts were higher in only two other years, 1997 and 2011. In 2011, a record 61 million acre feet of water washed out of the upper watershed. In 1997, the total was 49 million acre feet. This year’s forecast is for 41.4 million acre feet.
By holding back water and then doling it out, as the corps is doing now, the agency is preventing widespread flooding. Absent the upstream dams, the Missouri River valley would naturally flood because of the runoff in the upper watershed.
Reservoir levels peaked in July. The corps’ goal is to have reservoir levels down to their normal wintertime low before next year’s runoff season.
As a result of the increased water flowing into the river, the corps has been able to extend the guaranteed barge season by 10 days, with it ending Dec. 11 at the mouth of the river in St. Louis. Additionally, the corps has been able to generate additional electricity from its dams.
The reasons behind the high runoff are heavy snowpack last winter and subsequent heavy rains. On May 1, snowpack was 135 percent of the average above Fort Peck Dam in Montana and 129 percent of the average between Fort Peck and Garrison Dam in North Dakota.
This winter could be a different story. The U.S. Climate Prediction Center has said the odds favor El Niño conditionsacross North America. El Niño is a global weather pattern that originates with warmer than normal waters in portions of the Pacific Ocean. In a typical El Niño, the northern Great Plains receive less precipitation than normal. If that’s the case, the corps might not have to contend with such heavy runoff from snow next spring.
This year’s near-record runoff has occurred despite persistent drought in Montana. That’s not surprising. Water specialists and climate change scientists have noticed that the upper part of the watershed is developing a split personality: The west side of the basin is drier than normal, and the east side is wetter than normal.
Overall, most of the basin is seeing an increase in precipitation, according to the National Climate Assessment, the federal government’s authoritative report on the effects of climate change. In general, the trend in the basin is for the greatest increases in precipitation to occur in the spring and fall.
Additionally, extreme precipitation — deluges of snow or rain — is on the increase in the basin. The northern Plains have seen a 20 percent to 30 percent increase in the amount of precipitation falling during somewhat routine heavy storms — those storms that straddle two days and generate rain or snow amounts expected only once every five years. That doesn’t surprise atmospheric scientists because they’ve found that the planet is becoming more humid as it warms.
Mark T. Anderson, a scientist emeritus at the U.S. Geological Survey who has spent years studying the Missouri River, said the entire basin has seen marked changes over the past 60 years.
The headwaters in the mountains of Montana and Wyoming are seeing a decrease in precipitation, which is critical because that runoff sustains the base flow of the Missouri. On the other hand, atmospheric rivers are shuttling more moisture from the oceans into the heart of North America. The reasons behind these changes are ripe for more research, Anderson said.
“It’s a pretty well-established trend,” he said, adding that he couldn’t point to a single cause or say how long these conditions will persist.
Climate change and its associated impacts on precipitation has not yet been factored into management of the river, said John Remus, chief of the corps’ Missouri River management division.
“If climate change would cause a significant change in the amount or timing of runoff, we would assess a change in operations,” he said. “As it stands now, that has not been determined.”